The bombing killed 40 paramilitary police in India-governed Kashmir on Feb. 14. A Pakistan-based militant group claimed responsibility for the attack, sparking outrage in India that its neighbour would allow such a group to operate within its borders. India retaliated against the militants by launching an airstrike on one of the group’s camps inside Pakistan on Tuesday, violating the Pakistani government’s airspace in the process.

Politicians in both countries are now vowing to stand up to foreign aggression, while Pakistani and Indian newspapers run headlines warning about the potential for war.

“I vow that I will not let the country bow down,” Indian Prime Minister Narendra Modi told a gathering of former soldiers after the airstrike on Tuesday. Modi is up for re-election later this year, and his Hindu nationalist government has vowed a “jaw-breaking response” to the bombing.

Pakistani Prime Minister Imran Khan has denied any involvement in the attacks and offered to help investigate. He has also authorized the military to “respond decisively and comprehensively to any aggression or misadventure” from India. Pakistan’s president, Arif Alvi, warned on Tuesday that rhetoric “can lead to war.”

Here’s why the two sides are butting heads again in a conflict as old as Pakistan itself.

Kashmir caught in between

The British gave up control of India in 1947, and the country was split into two states: the Hindu-majority nation of India to the south, and the Islamic Republic of Pakistan to the northwest. More than 14 million people were forced to migrate across the border during the partition process, which was marked by ethnic and religious violence that caused between 1 and 2 million deaths.

In this September 1947, file photo hundreds of Muslim refugees crowd on top a train leaving New Delhi for Pakistan.

AP Photo

Caught in the middle of the partition was Jammu and Kashmir, a Muslim-majority state on the border between Pakistan and India. The Brits allowed the state, commonly known as Kashmir, to decide which country it would join. The case sparked war between Pakistan and India, and Kashmir’s status has been in dispute ever since.

WATCH: India and Pakistan on the brink of all-out military conflict

Both India and Pakistan claim to own Kashmir entirely, and two of their three wars have been over control of the region. India currently governs the central and southern parts of Kashmir, while Pakistan controls the northwestern area and China holds sway along its northeastern edge.

The disputed state is a mountainous region spanning approximately 225,000 square kilometres, making it roughly the same size as Idaho and half the size of the Yukon. India governs 45 per cent of the region, while Pakistan administers 35 per cent and China controls 20 per cent.

Although India holds the largest share of the territory, many of the citizens under its control are not happy with the state of affairs.

India-administered Kashmir has faced decades of conflict and violent insurgency from Muslim groups who seek independence or closer ties with Pakistan. More than 68,000 people are thought to have died since the insurgency broke out in 1989.

The Indian government has staged a series of crackdowns against these rebels over the years. The latest campaign rounded up approximately 400 suspected Kashmiri leaders and activists. Human rights groups say India has been responding to public protest with disproportionate force.

The Indian- and Pakistan-administered parts are split by a UN-monitored boundary called the Line of Control (LoC), where both sides are expected to obey the terms of a ceasefire. However, the two sides have fought several skirmishes over the years, and frequently accuse one another of violating the terms of the ceasefire.

India has also accused Pakistan of supporting Muslim insurgents in Indian-administered territory. Pakistan denies the accusations, saying that it offers only political and diplomatic support to the Kashmiri people in their struggle for self-determination.

Both countries field thousands of troops along the border.

The suicide bomb that reignited tensions

At least 40 paramilitary police officers were killed in Indian Kashmir on Feb. 14, when a suicide bomber drove a car into a military convoy. The Jaish-e-Mohammed (JeM), a Pakistan-based Muslim militant group, claimed responsibility shortly after the bombing. It was the worst attack since the Kashmir insurgency began in 1989.

WATCH BELOW: Dozens of police killed in suicide attack in Kashmir

The Pakistani government condemned the bombing and denied any involvement. However, India and the U.S. have suggested that Pakistan must do more to prevent such groups from flourishing within its borders.

“Pakistan must crack down on JeM and all terrorists operating from its territory,” U.S. National Security Adviser John Bolton tweeted on Feb. 16.

I expressed condolences to NSA Doval yesterday for the reprehensible terrorist attack on India. Pakistan must crack down on JeM and all terrorists operating from its territory. Countries should uphold UNSC responsibilities to deny safe haven and support for terrorists @nsaajit

India’s first public reaction to the attack was to withdraw the most-favoured nation trade status given to Pakistan and take all possible diplomatic steps “to ensure the complete isolation from international community of Pakistan.” India has also halted a key bus service with the Pakistani-controlled part of Kashmir.

WATCH BELOW: Hundreds of trucks stranded at India-Pakistan border

New Delhi said “incontrovertible evidence is available of (Pakistan) having a direct hand in this gruesome terrorist incident.”

Pakistan’s foreign secretary rejected the allegations, saying it was part of India’s “known rhetoric and tactics” to divert global attention from its human rights violations.

The two sides have recalled their ambassadors and traded threats and insults since Feb. 14. Meanwhile, citizens in India were clamouring for revenge.

India strikes back

India announced on Tuesday that it had launched an airstrike against a JeM camp inside Pakistan, near the town of Balakot.

The incursion into Pakistani airspace marks the deepest cross-border raid by India since the last of its three wars with Pakistan in 1971.

“The existence of such training facilities, capable of training hundreds of jihadis, could not have functioned without the knowledge of the Pakistani authorities.”

Pakistani military officials disputed India’s account of the strike, saying that the Pakistani Air Force chased away the Indian warplanes and forced them to release their payloads “in haste, while escaping.” Pakistan also denied harbouring JeM.

Indian aircrafts intruded from Muzafarabad sector. Facing timely and effective response from Pakistan Air Force released payload in haste while escaping which fell near Balakot. No casualties or damage.

Water war

In addition to its military response, India says it’s preparing to divert its share of the rivers that flow into southern Pakistan. The two nations share control of the rivers under the Indus Water Treaty.

“We will divert water from Eastern rivers and supply it to our people in Jammu and Kashmir and Punjab,” Nitin Gadkari, India’s minister of water resources, tweeted on Feb. 21.

The construction of dam has started at Shahpur- Kandi on Ravi river. Moreover, UJH project will store our share of water for use in J&K and the balance water will flow from 2nd Ravi-BEAS Link to provide water to other basin states.